hhiiilH ho/ 1 !/ tuMaal/ chare r\icf*f\\tat*
build, hack, tweak, share, discover,-
Written By: Terry Noe
Junction diode (1)
part reference D1 in schematic. All parts are available from Mouser Electronics
Battery connector (1)
Mouser #652-3386H-1-502LF. R1-R2
Hex inverter (1)
Mouser #511 -4069U. U1
Mouser #51 1-4070. U2
C2. C4-C6. C8
• Capacitor (1)
© Make Projects www.makeprojects.com Page 1 of 6
Banana jacks (2)
red J1 and black J 2
Project enclosure (1)
By Christie, Jim, John, and Terry Noe
Our dog, Maggie, loves to run around in our big yard. But how do we fence her in? Building a
500-foot fence is expensive and unsightly, so we use an "invisible fence." This is a buried
wire that runs around the edge of our yard. Maggie wears a radio collar that beeps when she
gets close to the wire, or gives her a mild shock if she strays over the edge (it doesn't hurt).
© Make Projects www.makeprojects.com Page 2 of 6
One of Maggie's favorite sports is digging up the gophers that are endemic to our
neighborhood. Unfortunately for us, the gophers have a hobby of their own, which is biting
through the buried wire of our invisible fence.
How do you find a break in a 500-foot buried wire? Digging up the entire 500 feet would not
be fun. You can buy commercial cable finders that will find buried wires and wire breaks, like
the Armada Technologies Pro 700, but these cost from $500 to $2,000! We decided to make
our own and use the money we saved for something useful, like building a big trebuchet.
© Make Projects www.makeprojects.com Page 3 of 6
Step 1 — Here's how it works.
• Our WireFinder 9000 has 2 components: a transmitter that puts a radio signal on the
buried wire, and a radio receiver. You connect the transmitter to one end of the buried
wire. This causes a current to flow through the wire, which creates a radio signal that
radiates from the wire. Then you can find the wire by walking around with the radio
• When the receiver is close to the buried wire, the radio signal is strong and you can hear it
over the receiver's speaker. The strength drops rapidly as you move away from the cable,
causing the signal to drop in volume until it becomes inaudible.
• If there's a break in your wire, the current can't reach the isolated segment. So when you
move past the break, the signal level drops off rapidly. Using the WireFinder, we were able
to locate the break in our wire quite accurately: the place we started digging was only 2
feet from the break.
• This technique works for finding both simple wires — like our invisible fence — and buried
coaxial cables as well.
• To keep things simple and cheap, we chose a regular AM radio as the receiver. Then we
designed our transmitter to generate a signal of the right frequency and modulation type so
that an AM radio can pick it up. Fortunately, the AM radio band is very low-frequency,
which makes construction and wiring very simple. The total cost of the transmitter is about
© Make Projects
Page 4 of 6
Step 2 — Construct the WireFinder.
• Build the transmitter circuit as shown in the schematic diagram. Standard wiring and
construction techniques can be used. Because AM radio operates at a relatively low
frequency, high-frequency construction techniques, such as controlled-impedance lines or
coaxial cables, are not required. The second photo shows how we arranged the
components on the protoboard.
• There are 2 adjustments you can make in the transmitter circuit. Potentiometer R1
controls the transmit frequency, and potentiometer R2 controls the pitch of the tone you
hear on your AM radio.
• First, center both pots in the middle of their adjustment range. Tune your AM radio to a
vacant frequency at the low end of the tuning range (we chose 574kHz) and place it near
the transmitter. Now adjust R1 while listening to the radio. When you tune the transmitter
to the same frequency, you'll hear a constant tone in the speakers.
• Adjust R2 to change the pitch of the tone to whatever you prefer; we tuned ours to 800Hz.
© Make Projects
Page 5 of 6
Step 3 — Use the WireFinder.
• To find your buried wire, connect one end of the wire directly to the red terminal, J1 , on the
WireFinder. You might find an alligator clip convenient. If you're trying to find a buried
coaxial cable, connect J1 to the outer braid or shield of the coax.
# Connect the black terminal, J2, to earth ground. The easiest way is to drive an aluminum
tent stake into the ground, and connect J2 to the stake. Don't use a painted stake; the
paint will prevent you from making an electrical connection. If your soil is very dry, you
can improve your ground connection by watering the soil around the stake.
• Turn your AM radio on and tune it to the frequency of the transmitter. Holding it close to
the wire, you should be able to hear the tone. Now you simply follow the wire using the AM
radio. If the signal level drops, you've either strayed from the wire path or you've passed
the break in the wire.
# Eventually, as you follow the wire, you should reach a point where the signal strength
drops off in every direction except back the way you came. Now get out your shovel and
start digging. The break in the wire should be nearby.
This project first appeared in MAKE Volume 20 , page 141 ,
This document was last generated on 201 2-1 1 -02 1 1 :56:46 PM.
© Make Projects
Page 6 of 6